The present invention relates generally to electrical discharge machining apparatus, and more particularly, to a process control system for controlling the electrical discharge machining apparatus to perform a complete machining of a workpiece.
Electrical discharge machining apparatus operates to machine an electrically conductive workpiece by causing intermittent electrical discharge across a gap between an electrode and the workpiece. An optimum clearance or working gap must be maintained between the electrode and the workpiece in order to enable the conduction of a suitable machining spark for properly eroding the workpiece. Presently, electrical discharge machining (EDM) apparatus is equipped with various controls which are individually adjusted to establish the various EDM parameters such as the on time for the electrical discharge across the gap, the period of time between electrical discharges, gap voltage, the polarity of the gap voltage and the other EDM parameters which must be controlled in the machining of a workpiece. Moreover, the machining of a single workpiece may require operator intervention at some point prior to the completion of machining to add a parameter which could not be set prior to the initiation of machining.
When the optimum EDM parameters are developed for a particular machining process, the settings on the EDM apparatus are noted and manually recorded. If at a later date the same machining process is employed, the operator refers to the recorded information in setting the controls. At best, this process is time consuming involving recentering steps and prone to human error either in the original recording of the EDM parameter settings of the particular machining process or in the resetting of the EDM apparatus for replicating the original machining process. Accordingly, absolute repeatability of the machining process cannot be assured in this type of EDM system. Additionally, valuable production time is wasted in manually resetting the EDM parameters for the repeat machining of a workpiece.
Another area of EDM processes which can be improved concerns the relatively slow movement of the electrode from a start-rest position to a machining position and the slow retraction of the electrode after machining is completed. In high volume production settings, rapid non-machining movement of the electrode would decrease the machining cycle time with cost savings accruing to the user.
It is the general object of this invention, therefore, to provide a process control system in electrical discharge machining apparatus for retaining operation instructions and controlling the machining of a workpiece in accordance with the operation instructions.
It is another object of the present invention to provide a process control system for an electrical discharge machining apparatus having storage means for retaining a plurality of operation instructions representing the machining of a workpiece to provide the electrical discharge machining apparatus with absolute repeatability in the machining of a plurality of workpieces.
It is another object of the present invention to provide a digitally controlled servo system and gap-sensing system cooperating to provide for an initial rapid advance of the electrode toward the workpiece and in response to the detection of the first electrical discharge across the gap to reduce the rate of electrode advancement to a slower feed rate.